1. Field of the Invention
The present invention relates to an image information processing method and an apparatus therefor, and, in particular, to such method and apparatus for converting binary image data into ternary or multi-level image data.
2. Related Background Art
Conventionally, an input device for entering image information to a digital image data processing apparatus, particularly a copying machine, a facsimile or a personal computer achieves image reading with a high resolving power and with satisfactory tonal rendition through the use of a solid state image sensor represented by a CCD. In the case of reproducing the original image from thus read image data with a binary printer, there is usually employed a suitable binarizing method according to the nature of the image, for example a binarizing method with a low-frequency threshold value for an image area of line-tone images, and a binarizing method with a high-frequency threshold value (pseudo-continuous tone reproduction) as represented by the dither method for an image area including continuous tone.
Such data compression/expansion technology is based on a concept, for an original image containing both line and continuous tone images, i.e., an original image containing both characters or lines and continuous tone image, of sacrificing the tonal information for an image area of characters and lines where emphasis should be given to the line-tone information, and sacrificing the line-tone information for an image area including continuous tone where the emphasis should be given to the tonal information. Consequently, in the case of reproducing the original image with a multi-level printer capable of recording three or more levels for each pixel, the image quality appears macroscopically satisfactory but appears reduced as the resolving power of the printer is improved, due to low resolving power in the tonal image area and high image contrast in the line-tone image area.